Last summer I spent three weeks in North Carolina as an apprentice with a timber framing company. I hope these pictures come out okay, I was using black and white film.
(https://forestryforum.com/images/04_01_03/cowee1.jpg)
Timber framed bridge with benches...nice place to sit and relax.
(https://forestryforum.com/images/04_01_03/cowee2.jpg)
Another apprentice working on a post for the house frame we were building.
(https://forestryforum.com/images/04_01_03/cowee3.jpg)
Workers loading a timber for me to cut out a 22ft. collar tie.
(https://forestryforum.com/images/04_01_03/cowee5.jpg)
Finished house timbers waiting in the yard.
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This was waiting for me when I got back to Pennsylvania 8)
Picture of the timber framed shelter another apprentice and I visited along the Appalachian Trail in Georgia. The sign on the shelter says: Chattahoochee National Forest, Plumorchard Gap Shelter
(https://forestryforum.com/images/04_01_03/shelteropt.jpg)
Nice pics! Brings back the memories. ;D
I have to ask, what kind of span has a 22ft collar tie?
I have to ask, whats a collar tie?
Jeff, a collar tie is what holds a pair of rafters together, so they don't spread apart under a roof load. If the rafters make an upside-down V, then the collar tie would make it look like an "A"
This was a really big house that we were constructing. I wish I had the square footage on the thing....it was only a one story house with a 4/12 pitch roof. The collar ties had three inch thick tenons on them that went into mortises in the rafters (the mortises were a PAIN to make).
How about a new thread here with a definition on some timber framing terms. A short Glossary if you will. I have seen several terms here already I don't know but sure would like to. :)
Have you ever been to the Woodhouse plant up in Mansfield, PA? I was there a number of years ago. They were working within tolerances of .003.
http://www.woodhouse-pb.com/
If you are interested in a house raising, they will be having them this summer in Centre Hall, Roaring Branch, and Williamsport.
For you Michigan folks, there will be one in Traverse Bay & Northport
We have a post and beam crafter in our area that supposedly showed these guys how to build post and beam. Last year I cut about 35 Mbf of white pine ranging in size from 6 x 8 to 10 x 10.
Thanks for the info. Ron! I've never been to this place, but it is worth checking about. Centre Hall is just over the mountain 8)
Oh, while I'm thinking about it...I remember why that collar tie was so long--it was part of a king-post truss system. Was racking my brain trying to remember the frame, and then I recalled making a through-mortise in the center of the timber for the king-post. I don't know if they're technically collar ties in this situation, since they meet the rafters not too far above where the rafter meets the post. So the collar tie is spanning almost the entire width of the bent.
Jeff, we'll have to work on the glossary....maybe I'll start it when I'm not at work ::) :D
Ron, I think that raising schedule was for last summer :'(
Great pictures Ohsoloco. You must learn quite abit in the three week class. The intro I took was for only a week, and I learned an awfully lot.
I think what you refer to as a collar tie would be refered to as the bottom chord of the king post truss. And most collar tie's usually act in compression, actually keeping the rafters from sagging from the roof load.
Did you get that Jeff? :D
Actually, I didn't take a "class." I signed up for an apprenticeship, which basically meant that I worked there for free, and they gave me a place to stay while I worked there :)
I did get time in the evenings to work with one of the engineers, who was a lot of help. She refreshed my memory of some algebra to help me with frame design, joinery layout, timber lengths, etc.
I call it a collar tie, cuz when I was working on the timber, it was named CT(some number)--the CT denoting collar tie. I mean, it is keeping the rafters from spreading....
sawinmontana, do collar ties work in compression? When I think of compression I think of the top half of a timber that has a load put on it....the bottom half being in tension. Could you explain this a little more? I may know a few things, but I'm certainly no eggspert :D
When I think of compression I think of the top half of a timber that has a load put on it....the bottom half being in tension.
Your description is pretty descriptive of a beam loaded in bending (the beam being supported at its ends while supporting a load towards its middle), where the "top" of the beam is in compression and the "bottom" is in tension.
A load pushing on the ends of a piece puts that piece in compression (parallel to the grain).
Pulling on the ends will put a piece in tension (parallel to the grain).
I'm no expert either. I just read enough and comprehend a little to be very dangerous!
Beenthere, you explained that pretty good. I think that is what they refer to as horizontal shear.
Ohsoloco, I bought a book(agin reading=danger) called Timber frame joinery and design handbook from the Timber Framers Guild, its really full of information. I thought like you that a collar tie had to be resisting the outward thrust of the rafters, but the walls are doing that. Even a roof that used a supported ridge beam so the rafters hung on the ridge and sat on the wall, no outward thrust, may still use a collar tie to support the rafter against sag. I don't think I'm doing a very good job of expaining it. I guess it would also depend on where on rafter it was located. When I think of a collar tie it would be in the middle or slightly above on the rafter. If you lowered it towards the feet of the rafter then it may go into tension. In the book I mentioned above there is quite an article this subject.
Scott
I was confused about this, because I was thinking that a collar tie would be in tension, since the rafters want to push out, and the collar tie is keeping it from doing that.
I know what you mean about reading...just enough to keep me dangerous, too :D
I decided to refer to one of my books last night ;) I'm even more confused now ::) Okay, this was in "The Timber Frame Home" by Tedd Benson. He refers to a collar strut, and a collar tie. The collar strut is in compression b/c it is keeping the rafter peaks from pushing into each other too much, and the collar tie is in tension b/c it is keeping the rafter feet from pushing out the walls. Okay, I buy that....but they illustrated it with pictures, and the picture for the collar tie and the collar strut are exactly the same except for the direction the arrows are pointing (which indicate a force being applied to the timber). I'd like to hear from anyone else about this...I'm confused :-/
I was thinking the same as you, sawinmontana...that if the collar tie (or collar strut in this case) was high up on the rafters, then it would be in compression, and if it was lower, then it would be in tension. The illustration showed them in the same spot.
I think I'm confused now also!! I believe we're thinking about the same. I guess it depend if the walls had a connecting girt or not. In the book Timber Frame Joinery and Design there is an article by Ed Levin, he made a sample bent on a computer and it showed all the loads. that bent was a high posted cape, with about 3 foot knee wall, the posts were connected by a girt. in his model he had to much pressure or out ward thrust a the rafter feet, but the collar tie he had was still acting in compression. He kept lowering it to make it come into tension to take some of the load and he had to get it to around 2 feet or so of the rafter feet before it went into tension. It is real interesting article, the whole book is full of interesting stuff. I'd recomend it to any one interested in cutting a frame.
Next time I'm in the big city I'm going to ask the professional timberframers I know about this subject.
Scott
I should check that book out. Ed Levin seems to know his sh...tuff. He makes some beautiful frames (and some downright complicated ones at that!).